The invention relates to 2-dimensional position sensors. More particularly the invention relates to 2-dimensional position sensors of the type based on capacitive proximity sensing techniques. Such sensors may be referred to as 2-dimensional capacitive transducing (2DCT) sensors. 2DCT sensors are based on detecting a disturbance in a capacitive coupling caused by the proximity of a pointing object. A measured location for the disturbance corresponds to a measured position for the pointing object.
2DCT sensors are typically actuated by a human finger, or a stylus. Example devices include touch screen and touch sensitive keyboards/keypads, e.g. as used for controlling consumer electronic devices/domestic appliances, and possibly in conjunction with an underlying display, such as a liquid crystal display (LCD), or cathode ray tube (CRT). Other devices which may incorporate 2DCT sensors include pen-input tablets and encoders used in machinery for feedback control purposes, for example. 2DCT sensors are capable of reporting at least a 2-dimensional coordinate, Cartesian or otherwise, related to the location of an object or human body part, by means of a capacitance sensing mechanism.
Devices employing 2DCT sensors have become increasingly popular and common, not only in conjunction with personal computers, but also in all manner of other appliances such as personal digital assistants (PDAs), point of sale (POS) terminals, electronic information and ticketing kiosks, kitchen appliances and the like. 2DCT sensors are frequently preferred to mechanical switches for a number of reasons. For example, 2DCT sensors require no moving parts and so are less prone to wear than their mechanical counterparts. 2DCT sensors can also be made in relatively small sizes so that correspondingly small, and tightly packed keypad arrays can be provided. Furthermore, 2DCT sensors can be provided beneath an environmentally sealed outer surface/cover panel. This makes their use in wet environments, or where there is a danger of dirt or fluids entering a device being controlled attractive. Furthermore still, manufacturers often prefer to employ interfaces based on 2DCT sensors in their products because such interfaces are often considered by consumers to be more aesthetically pleasing than conventional mechanical input mechanisms (e.g. push-buttons).
US 2008/0246496, published on 9 Oct. 2008 describes a 2DCT sensor comprising a substrate with a sensitive area defined by a pattern of electrodes. The 2DCT is of the so-called “active” or “mutual” type in which proximity of an object is sensed by the changes it induces in coupling between a drive electrode and one or more adjacent sense electrodes. Measurement of the coupling is carried out by applying a transient voltage to the drive electrode and making a measurement of the capacitance between the drive and associated sense electrode(s) that results.
In the specific prior art design of US 2008/0246496 the drive and sense electrodes are arranged in a single layer on one side of a substrate—typically the underside of a touch panel made of glass or a plastics material. Having a single layer electrode pattern is a generally desirable feature, since it provides simplicity, lower cost and a low profile.
FIG. 8 of the accompanying drawings illustrates schematically a representative portion of the prior art electrode pattern of US 2008/0246496. The electrodes 12 are drive electrodes represented by longitudinal bars extending in the x-direction. Adjacent drive electrodes 12 are spaced apart by three groups 2, 4, 6 of sense electrodes which are tapered in the x-direction so that permutations of 2 out of the 3 groups co-extend over different portions of the lateral extent of the panel. Each central sense electrode 4 is connected to an external circuit connection S2 via a conductive trace 5 that divides the adjacent side sense electrode 6 into two portions 6A and 6B separated by a channel, with an additional trace 3 extending to separate the left-hand portions 2A and 2B of electrode 2. The side electrodes 2 and 4 are connected to the external circuit via connections S1 and S3 respectively. In use, the position of an object is determined in a measurement acquisition cycle in which the bar electrodes 12 are sequentially driven by respective drive channels and the amount of charge transferred to the sense electrodes from each bar electrode is determined by the sense channels.
While providing good resolution in the x-direction a consequence of this design approach is that each group of sense electrodes has a significant vertical thickness, i.e. dimension in the y-direction. The vertical repeat period dimension P is therefore quite large and difficult to reduce owing to the complexity of the sense electrode patterning. For a finger actuated device, as long as the dimension P is comparable to a finger press dimension, there is no problem with vertical resolution. However, a problem does arise when there is a desire to operate the 2DCT in multi-touch mode.
WO 2009/00704A1 describes a 2DCT capable of detecting multiple simultaneous touches, as well as summarizing the prior art multi-touch approach of U.S. Pat. No. 5,825,352. WO 2009/00704A1 detects multiple simultaneous touches using a development of the technique described in U.S. Pat. No. 6,993,607. The touch panel of WO 2009/00704A1 delivers sets of capacitance signal values to a processor which computes the coordinates of single or multiple touch locations on the touch panel. The processing of each set is performed by (i) identifying the sensing element having the largest capacitance signal value; (ii) defining a region around that sensing element; and (iii) repeating the process iteratively, wherein each subsequent identifying step excludes signals that lie in previously defined regions. A multi-touch sensor is thus provided in which the signal processing is based on successive definition of regions or sub-blocks in the touch panel. Depending on the processing carried out, to be resolvable simultaneous touches must be separated by one or two clear sub-blocks (using the language of WO 2009/00704A1) or drive/sense electrode units (using the language of the present application).
Consequently, to distinguish between multiple simultaneous touches in a 2DCT made up of a plurality of drive/sense electrode units extending in the x-direction, the touches need to be separated in the y-direction by at least 2, in practice probably at least 3, drive/sense electrode units. So, for example, if a touch screen or touch panel has a limited vertical dimension of say 60 mm, and the electrode units have a vertical dimension of 10 mm, then a maximum of 6 electrode units will fit in, so the screen will only be able to detect up to 2 or 3 simultaneous touches, depending on their position on the screen. On the other hand, if the electrode units have a smaller vertical dimension of say 6 mm, then 10 of them can be provided, and the screen will be able to detect up to 3 or 4 simultaneous touches.
It would therefore be desirable to provide a single-layer electrode pattern suitable for a 2DCT of the active or mutual type that can be embodied with a relatively small vertical repeat period dimension for the drive/sense electrode units.